In this description, reference may be made to "profiles", "rolled steel sections", and like structural elements suitable for use as beams, guiders, columns and the like. While in most cases the structural shapes involved will be those generally described as I-beams, H-beams and flanged beams, in most cases the structural shape will have a web which generally lies horizontally during drilling, will be provided with at least one flange along at least one edge of the web and will be displaced through the machine on a roller conveyer or table in a prone or recumbent position so that the height H of the structural shape is measured horizontally.
The German patent publication-open application DE-OS No. 31 36 394 describes such a drilling apparatus in which the workpiece lies on a roller table with its height extending in horizontal direction.
The drilling apparatus includes a frame with an upright movable in horizontal direction towards the workpiece to define a clamping path in cooperation with movable press rollers which press the workpiece against transport rollers located opposite to the press rollers. The opposite flanges of I-beam and H-beam sections are engaged by these rollers. The movable upright is connected to a stationary upright via a transverse beam along wich a drilling unit is moved. The drilling unit is operated by a control unit so as to be positioned in reference to the centerline of a standard workpiece of nominal or standard height.
For correcting the position of the drilling unit in the event the actual height of the workpiece deviates from the set nominal height a possible shift of the reference line will occur to cause the program of holes to deviate from their desired positions vis-a-vis the reference line. Accordingly, this earlier system uses as its essential element a Bowden cable one end of which is connected to a control cam and whose other end is linked to a double-armed angle lever which is connected to the bearing for the press rollers. The lever assembly transmits the movement of the bearing to the control cam by a ratio of 1:2.
While this principle is sound as the Bowden cable follows the movement of the movable upright and press rollers in accordance with the height tolerance of the rolled steel section during clamping of the latter, the accuracy of transmitting the movement from the bearing to the control cam is not entirely satisfactory so that the web bores provided by the drilling unit on the rolled steel sections do not always correspond to a desired drilling pattern.
Reference is made herein to the centerline as a representative baseline used as a refernce point for control of the drilling pattern. Naturally, the two-to-one transmission ratio between the displacement of the movable upright through the distance .+-.h compensating for height variations in the structural section and the correction correspondingly of .+-.h/2 of the reference position of the drilling head will always automatically align the head with the centerline within the limits of accuracy as noted, but this system is totally unsuited to any other baseline which may be required, such as an edge of the section.